In general, the proper operation of a tape or web transport system in which the supply and take-up reels are simultaneously driven by surface engagement with a driving capstan requires the presence of tension in the tape span between the supply and take-up reels. This tape tension permits the formation of a tape pack which will retain its physical shape without side support and which will also withstand the high speeds and rapid accelerations and decelerations normally associated with units of this type. In addition, it is desirable that the tension be maintained at a constant level in order to assure uniform contact between the recording or sensor head and tape surface so as to enable the transduction of a flutter-free signal.
An example of such a type of transport is disclosed in U.S. Pat. No. 3,921,933, the disclosure of which is incorporated herein by reference. The constant tape tension in that tape transport is maintained almost entirely by braking the reel serving as the supply reel. The braking causes the surface velocity of the capstan and that of the unbraked take-up reel to slightly exceed the surface velocity of the supply reel. To accommodate the difference in speed between the supply and take-up reels the interconnecting span of tape is forced to stretch, thus generating the desired tape tension.
As an alternative to supply reel braking, other tape transports have relied upon the differential deformation or progression phenomenon. The device disclosed in U.S. Pat. No. 3,370,803 is an example of such a tape transport. Tape tension is generated by using a capstan having a peripheral portion capable of elastic flow. By applying a greater force at the take-up reel/capstan interface than at the supply reel/capstan interface, the rate of local acceleration by elastic flow of the capstan material at the former will exceed that at the latter. The effect is to impart a slightly higher velocity to the take-up reel with the result that tension is generated in the tape between the supply and the take-up reels by introducing, as with supply reel braking, a velocity difference between the take-up and supply reels. However, it has been found that with both of these prior art methods of tape tensioning serious difficulties have been encountered in achieving constant tape tension throughout the entire transport operation. The primary source of these problems being the undesirable side effects caused by the continual change in the supply and take-up reel size diameter as the transport operation is carried out.
Therefore, in an attempt to overcome these difficulties, various methods have been developed combining the effects of progression and supply reel braking so as to achieve constant tape tension throughout the tape pack. For example, in U.S. patent application Ser. No. 574,958, filed May 6, 1975 now U.S. Pat. No. 4,018,402, the disclosure of which is incorporated herein by reference, the progression effect was combined with the effect of an automatic braking system wherein the braking force applied to the supply reel varied as a function of reel size diameter. However, in practice it has been found that this variable braking system is both expensive to manufacture and complicated to set up. These drawbacks are primarily attributable to the fact that a pair of unidirectional clutches are preferably utilized in the system which operate in conjuction with springs which must be precise in their operation over a wide extension distance. Furthermore, these springs must be balanced with respect to each other so as to accommodate the bidirectional operation of the machine.
An alternative method combining the effects of progression and supply reel braking to achieve constant tape tension throughout the tape pack is described in U.S. patent application Ser. No. 574,959, filed May 6, 1975 now U.S. Pat. No. 4,049,216, the disclosure of which is likewise incorporated herein by reference. In that disclosure, constant tape tension was maintained primarily by placing a flexible, but relatively inelastic, belt around the periphery of the resilient capstan. However, due to the greater torsional stiffness of the capstan periphery caused by utilization of this belt, it is difficult to generate "creep" effects, discussed in detail below, which obviate the problem of tape looping encountered with loosely wound initial tape packs.